Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions to metal atoms on a substrate without application of an electrical current. Electroless plating processes and compositions are found in a wide variety of commercial practices and are used for plating a substantial number of metals and alloys onto various substrates. Examples of materials commonly plated through this process can include copper, nickel, gold, cobalt, tin-lead alloys, etc. The substrate surface can be any surface that is either catalytically active itself or can be activated by a catalyst. Possible substrates common in the past include, for example, metal, diamond, and a variety of polymers Plating processes can be either selective, i.e., only a portion of the substrate surface is catalytically activated to control precisely where metal deposition will occur, or alternatively can be used to coat an entire substrate surface.
Electroless plating has been widely used in the microelectronics industry for deposition of layers on semiconductor wafers. For example, electroless plating has been used in the past to form adhesion, barrier and capping layers on substrates. For the purposes of this disclosure, a barrier layer is defined as a layer formed on at least a portion of a substrate surface which can prevent contact between the materials located on either side of the barrier layer. For example, a barrier layer can prevent oxidation or otherwise render passive the material covered by the barrier layer, or alternatively can prevent the material contained in a layer located on one side of the barrier layer from diffusing into a layer located on the other side of the barrier layer.
Electroless plating processes known in the past generally include heating a bath solution to a certain deposition temperature, which generally corresponds to at least the minimum deposition temperature (i.e. the minimum temperature where deposition from that bath to that substrate can occur). After heating, the bath solution is pumped into a plating chamber. In the plating chamber, a substrate having an activated surface is present and the electroless plating begins at or near the time the hot solution contacts the substrate.
The plating process itself includes an induction period followed by a steady-state deposition period. The induction period is the time necessary to reach the mixed potential at which the steady-state metal deposition occurs. The deposition is usually designed to occur in a certain pH and temperature range.
One particular example of layers that have been formed using electroless plating are cobalt-tungsten-phosphorous (CoWP) films. Such films have proven to be an effective diffusion barrier for copper metalization, a capping layer to prevent copper from oxidation and have also shown to improve adhesion of the inlaid copper layer to an upper dielectric layer.
In the past, in order to form a CoWP layer on copper, the conventional process included two steps and two separate solutions. In the first step, the substrate being coated was placed in a palladium salt solution causing palladium to deposit on the copper according to a replacement reaction. In the second step, the palladium treated copper layer was then contacted with a second solution causing a CoWP alloy to form a layer on the substrate. The second solution normally contained sodium tungstate as the tungsten ion source, and potassium hydroxide or sodium hydroxide as a pH-adjusting agent. The palladium acted as a catalyst to initiate deposition of the CoWP alloy.
Unfortunately, the alkali metal ions (potassium or sodium) present in the second solution may act as main mobile ionic contaminants. In particular, the metallic ions can move inside the device, causing the device to fail.
In view of the above-described drawbacks of conventional electroless plating solutions, those skilled in the art have attempted to develop alkali metal-free solutions for forming electroless deposited CoWP layers. Although some alkali metal-free solutions have been developed, the solutions typically contain tetramethylammonium hydroxide (TMAH) as a strong alkaline source. TMAH, however, is very expensive dramatically increasing the cost of the electroless plating solution.
In view of the above, a need currently exists for an electroless plating solution that is alkali metal-free and economical to produce. More particularly, a need exists for an electroless plating solution capable of forming cobalt-tungsten alloys that is alkali metal-free and does not contain TMAH. A need also exists for an electroless plating solution that can deposit a cobalt-tungsten alloy layer on a substrate, such as a micro-electronic device, in a single step without having to first contact the substrate with a catalyst, such as a palladium salt.
In general, the present invention is directed to electroless plating solutions and to processes for using the solutions. The electroless plating solutions of the present invention are alkali metal-free. Further, in one embodiment, the solution can be free of tetramethylammonium hydroxide. In addition, in other embodiments, the solution can deposit an alloy layer on a substrate without first having to treat the substrate with a catalyst, such as a palladium salt.
For example, in one embodiment, the present invention is directed to an electroless plating process for forming a layer in a micro-electronic device. The process includes the steps of first providing an electroless plating solution comprising a reducing agent, a cobalt source, and a tungsten source. According to the present invention, the reducing agent comprises a borane-dimethylamine complex (DMAB).
The above electroless plating solution is contacted with a substrate to form a deposit on the substrate by electroless deposition. The deposit is a cobalt-tungsten alloy, such as a cobalt-tungsten-boron alloy or a cobalt-tungsten-phosphorous-boron alloy.
In addition to the above ingredients, the electroless plating solution can also contain a pH adjusting agent, a buffer, a complexing agent, a stabilizer, and one or more surfactants. Examples of pH stabilizers can include an amine, an ammonium hydroxide, or a hydroxy amine.
The buffer, on the other hand, can be boric acid or an ammonium salt. The complexing agent may be an amino acid, a hydroxy acid, or an ammonium salt thereof.
The cobalt source contained within the solution can be cobalt sulfate, cobalt chloride, or cobalt hydroxide. The tungsten source can be tungstic acid, tungsten oxide, ammonium tungstate, or phosphorous tungsten acid.
In this embodiment, the solution may be alkali metal-free, can contain no tetramethylammonium hydroxide, and can be used to form a cobalt-tungsten layer on a substrate without having to first contact the substrate with a catalyst, such as a palladium catalyst.
In another embodiment of the present invention, the electroless plating solution can include a reducing agent, a pH-adjusting agent, a cobalt source, and a tungsten source. In this embodiment, the reducing agent can be a hypophosphorous acid or an ammonium hypophosphite. Again, the electroless plating solution can be alkali metal-free and can contain no tetramethylammonium hydroxide.
In this embodiment, a cobalt-tungsten alloy layer can be formed with or without palladium activation. In order to eliminate the use of a palladium catalyst, the electroless plating solution can further contain a second reducing agent comprising a borane-dimethylamine complex.
In addition to forming cobalt-tungsten alloy layers, nickel-tungsten alloy layers, cobalt-rhenium alloy layers, and nickel-rhenium alloy layers may also be formed. For instance, when forming a nickel-tungsten alloy layer, the above cobalt source can be replaced with a nickel source. Examples of nickel sources include nickel hydroxide and various soluble nickel salts such as nickel sulfate, nickel chloride, and the like.
When forming a cobalt-rhenium alloy layer, the above-described tungsten source can be replaced with a rhenium source. Examples of rhenium sources include rhenium (VII) oxide, perrhenic acid, ammonium perrhenate, and the like.
In still another embodiment of the present invention, the above-described nickel sources and rhenium sources can be used to form nickel-rhenium alloy layers.
The above alloy layers can be formed on various substrates, such as on copper or on a low K dielectric material.
Other features and aspects of the present invention are discussed in greater detail below.